"And then let's just work on reducing that level of poverty, because it's wrong, it denies future life opportunities and it's something a rich country like ours ought to be able to do something about," Mr Little said.

Too difficult to measure - PM

Mr Key ruled out taking up the commissioner's challenge, saying it was too difficult to measure definitively the number of children in poverty and so a target could not be set.

It was better to focus on factors that contributed to deprivation rather than exact numbers, Mr Key said.

"Isn't it better for the government to say, 'Rheumatic fever's an issue, potentially prevalent with high levels of deprivation, and therefore let's focus on that', rather than worry too much about the individual measure of poverty?'"

Green Party co-leader Metiria Turei said it was inexplicable that Mr Key would not set a target, and described his refusal to commit to one as irresponsible.

"He may be refusing to set a target and clear measurements because he doesn't want to acknowledge the extent of the problem; that makes him a deeply irresponsible prime minister and unfit for the role," Mrs Turei said.

Government already has poverty targets

Meanwhile, the government has been reminded of its existing targets to reduce poverty.

Victoria University professor Jonathan Boston said the government was signed up to a target of reducing child poverty by 50 percent by 2030, under the United Nations Sustainable Development Goals it adopted last year.

"It's not a question of whether New Zealand should have targets or not, we already have them, the government has already signed up to them," he said.

"The real issue is, how are we going to deliver on those targets?"

When it adopted the Sustainable Development Goals in September 2015, the government agreed to "reduce at least by half the proportion of men, women and children of all ages living in poverty in all its dimensions, according to national definitions, by 2030".